Graded-refractive-index optical plastic material and method for its production

ABSTRACT

A graded-refractive-index optical plastic material composed of an amorphous fluoropolymer and at least one material which differs from the polymer in refractive index by at least 0.001, wherein the material is so distributed as to have a concentration gradient in a specific direction, and a method for its production.

TECHNICAL FIELD

[0001] The present invention relates to a graded-refractive-index optical plastic material (hereinafter sometimes referred to as optical plastic material for short) having high transparency and high heat resistance simultaneously, which conventional optical plastics hardly ever attained, and a method for its production.

[0002] The optical plastic material of the present invention may be a light transmission medium such as an optical fiber, or a body material of a light transmission medium such as a preform of an optical fiber.

[0003] A light transmission medium which is the optical plastic material of the present invention is free from light scattering and very transparent to light at wavelengths within a wide range from ultraviolet light to near infrared light, since it is made of an amorphous resin, therefore it is useful for optical systems for light of various wavelengths. In particular, the optical plastic material of the present invention provides a light transmission medium with small losses at wavelength of 1300 nm and 1550 nm, at which a trunk vitreous silica fiber is used in the field of optical communication.

[0004] A light transmission medium which is the optical plastic material of the present invention has heat resistance, chemical resistance, humidity resistance and nonflammability enough to withstand severe use conditions, for example, in an engine room of an automobile.

[0005] A light transmission medium which is the optical plastic material of the present invention is useful as various graded-refractive-index light transmission medium such as a graded-refractive-index optical fiber, a rod lens, an optical waveguide, an optical decoupler, a wavelength multiplexer, a wavelength demultiplexer, an optical attenuator, a light switch, an optical isolator, a light transmitting module, an light receiving module, a coupler, an optical deflector and an optical integrated circuit. Graded-refractive-index means a region wherein the refractive index of a light transmission medium varies continuously in a specific direction. For example, a graded-refractive-index optical fiber shows a refractive index profile that the refractive index parabolically decreases from the center of the fiber along the radii.

[0006] When the optical plastic material of the present invention is a body material of a light transmission medium, it is spun, for example, by hot drawing to prepare a light transmission medium such as a graded-refractive-index optical fiber.

BACKGROUND ART

[0007] Heretofore, as resins for graded-refractive-index plastic light transmission medium, optical plastics represented by methyl methacrylate resins, tetrafluoroethylene resins disclosed in WO94/04949 and vinyl fluoride resins have been proposed.

[0008] With respect to stepped-refractive-index plastic optical fibers, many proposals have been made to use optical plastics such as a methyl methacrylate resin, a styrene resin, a carbonate resin and a norbornene resin for a core and a fluoropolymer cladding. Japanese Unexamined Patent Publication No. 244007/1990 proposes use of a fluoropolymer core and a fluoropolymer cladding.

[0009] The present invention provides an optical plastic material having heat resistance, humidity resistance, chemical resistance and nonflammability required for applications to an automobile, an office automation (OA) equipment, an electrical appliance and the like, which light transmission medium made of a methyl methacrylate resin or a norbornene resin have never attained.

[0010] Further, the object of the present invention is to provide a novel optical plastic material which is useful for ultraviolet light (wavelength from 200 nm to 400 nm) and near infrared light (wavelength from 700 nm to 2500 nm), which are unavailable to light transmission medium made of a methacrylate resin, a carbonate resin and a norbornene resin, and has low light transmission losses in a wide transmission zone and a method of its production.

DISCLOSURE OF INVENTION

[0011] The present inventors have conducted extensive researches taking the above-mentioned problems into consideration, and consequently found that a fluoropolymer which substantially has no C—H bond is the most suitable to provide heat resistance, humidity resistance, chemical resistance and nonflammability and eliminate C—H bonds (namely carbon-hydrogen bonds) which absorb near infrared light. The fluoropolymer has C—F bonds (namely carbon-fluorine bonds) instead of C—H bonds.

[0012] When a substance is exposed to light, a certain interatomic bond absorbs preferentially light of wavelength resonant with its stretching vibration and deformation vibration. Conventional polymeric materials used for plastic optical fibers are mostly compounds having C—H bonds. Such polymeric materials which basically have C—H bonds show the main absorption bands at a shorter wavelength (3400 nm) in the infrared region, since a hydrogen atom is so light as to easily vibrate. Accordingly, in the near infrared to infrared region (from 600 to 1550 nm), which is the wavelength region of a light source, relatively lower harmonic absorption peaks due to the stretching vibration of C—H bonds appears at intervals and they are greatly responsible for absorption loss.

[0013] If hydrogen atoms are substituted with fluorine atoms, these harmonic absorption peaks shift to longer wavelengths, and the amount of absorption in the near infrared region decreases. In the case of a PMMA (polymethyl methacrylate) having C—H bonds, the absorption loss attributable to the C—H bonds is estimated theoretically to be 105 dB/km at a wavelength of 650 nm and at least 10000 dB/km at a wavelength of 1300 nm.

[0014] On the contrary, in the case of a material in which hydrogen atoms are substituted with fluorine atoms, there is substantially no absorption loss at a wavelength of 650 nm, and the absorption loss at a wavelength of 1300 nm, which is between the sixth and the seventh overtones, is in the order of 1 dB/km and therefore negligible. For this reason, we propose to use a compound having C—F bonds.

[0015] It is also preferred to eliminate functional groups such as a carboxyl group and a carbonyl group which inhibit heat resistance, humidity resistance, chemical resistance and nonflammability. Further, since the presence of a carboxyl group results in absorption of near infrared light, and the presence of a carbonyl group results in absorption of ultraviolet light, it is preferred to eliminate these groups. In addition, in order to reduce a transmission loss due to light scattering, it is important to use an amorphous polymer.

[0016] In the case of a stepped-refractive-index optical fiber, multimodal light is propagated in it, by being reflected on the interface between the core and the cladding. Therefore, mode dispersion occurs, and as a result, the transmission zone decreases. However, a graded-refractive-index optical fiber hardly causes mode dispersion, and therefore, the transmission zone increases.

[0017] The present inventors found out an optical plastic material composed of an amorphous fluoropolymer which substantially has no C—H bond, especially a fluoropolymer having a cyclic structure on its main chain, and a material which differs from the polymer in refractive index, wherein the concentration of the material shows a gradient in a specific direction and a method of its production for the first time, and achieved the following present inventions (1) to (2).

[0018] (1) A graded-refractive-index optical plastic material composed of an amorphous fluoropolymer (a) which substantially has no C—H bond and at least one material (b) which differs from the fluoropolymer (a) in refractive index by at least 0.001, wherein the material (b) is so distributed in the fluoropolymer (a) as to have a concentration gradient in a specific direction. (2) A method of producing a graded-refractive-index optical plastic material, which comprises melting an amorphous fluoropolymer (a) which substantially has no C—H bond, injecting at least one material (b) which differs from the fluoropolymer (a) in refractive index by at least 0.001, or the fluoropolymer (a) containing the material (b) at the center of the melt of the fluoropolymer (a), and molding the melt while or after diffusing the material (b) to form a region wherein the refractive index varies continuously. Heretofore, a tetrafluoroethylene resin, a perfluoro(ethylene-propylene)resin, a perfluoroalkoxy resin, a vinylidene fluoride resin, an ethylene-tetrafluoroethylene resin, a chlorotrifluoroethylene resin and the like have been widely known as fluoropolymers. However, since these fluoro resins are crystalline and causes light scattering, they are poor in transparency and unfavorable as materials for a plastic light transmission medium.

[0019] In contrast to these fluoro resins, amorphous fluoropolymers are free from light scattering due to crystals and therefore, are excellent in transparency. The fluoropolymer (a) of the present invention is by no means limited so long as it is an amorphous fluoropolymer having no C—H bond, however, a fluoropolymer having a cyclic structure on its main chain is preferred. As a fluoropolymer having a cyclic structure on its main chain, fluoropolymers having a fluorine-containing alicyclic structure, a fluorine-containing cyclic imide structure, a fluorine-containing triazine ring structure or a fluorine-containing aromatic ring structure are preferred. Among fluoropolymers having a fluorine-containing alicyclic structures those having a fluorine-containing alicyclic ether structure are more preferred.

[0020] A fluoropolymer having a fluorine-containing alicyclic structure is less likely to undergo orientation of polymer molecules, when hot drawn or melt spun into fibers, as compared with fluoropolymers having a fluorine-containing cyclic imide structure, a fluorine-containing triazine ring structure or a fluorine-containing aromatic ring structure. Consequently, it does not cause light scattering, therefore is a more preferred polymer.

[0021] The viscosity of the fluoropolymer (a) in a molten state is preferred to be from 10³ to 10⁵ poise at a melt temperature of from 200° C. to 300° C. If the melt viscosity is too high, not only melt spinning is difficult, but also diffusion of the material (b) required for a graded refractive index, hardly takes place, and formation of a graded refractive index is difficult. On the other hand, if the melt-viscosity is too low, there are practical problems. Namely, in the case of use as a light transmission medium in an electronic equipment or an automobile, it becomes soft upon exposure to a high temperature, and the light transmission performance becomes poor.

[0022] The number-average molecular weight of the fluoropolymer (a) is preferably from 10,000 to 5000,000, more preferably from 50,000 to 1000,000. A too small molecular weight can interfere with heat resistance, and too a large molecular weight makes it difficult to form a graded-refractive-index light transmission medium, such being unfavorable.

[0023] As a polymer having a fluorine-containing alicyclic structure, those obtainable by polymerization of a monomer having a fluorine-containing cyclic structure, and a polymer having a fluorine-containing alicyclic structure on its main chain which is obtainable by cyclic polymerization of a fluorine-containing monomer having at least two polymerizable double bonds are preferred.

[0024] Polymers having a fluorine-containing alicyclic structure on their main chains which are obtainable by polymerization of monomers having a fluorine-containing alicyclic structure are reported by Japanese Examined Patent Publication No. 18964/1988 and the like. Namely, polymers having a fluorine-containing alicyclic structure or their main chains are obtained by homopolymerization of a monomer having a fluorine-containing alicyclic structure such as perfluoro(2,2-dimethyl-1,3-dioxole), or by polymerization of such a monomer with a radical polymerizable monomer such as tetrafluoroethylene, chlorotrifluoroethylene and perfluoro(methyl vinyl ether).

[0025] Further, polymers having a fluorine-containing alicyclic structure on their main chains which are obtainable by cyclic polymerization of a fluorine-containing monomer having at least two polymerizable double bonds are reported by Japanese Unexamined Patent Publication No. 238111/1988, Japanese Unexamined Patent Publication No. 238115/1988 and the like. Namely, polymers having a fluorine-containing alicyclic structure on their main chains are obtained by cyclic polymerization of a monomer such as perfluoro(allyl vinyl ether) and perfluoro(butenyl vinyl ether), or copolymerization of such a monomer with a radical polymerizable monomer such as tetrafluoroethylene, chlorotrifluoroethylene and perfluoro(methyl vinyl ether).

[0026] Polymers having a fluorine-containing alicyclic structure on their main chains are also obtained by copolymerization of a monomer having a fluorine-containing alicyclic structure such as perfluoro(2,2-dimethyl-1,3-dioxole) with a fluorine-containing monomer having at least two polymerizable double bonds such as perfluoro(allyl vinyl ether) and perfluoro(butenyl vinyl ether).

[0027] As examples of the above-mentioned polymer having a fluorine-containing alicyclic structure, those having a repeating unit selected from the following formulae (I) to (IV) are mentioned. Part of the fluorine atoms in such polymers having fluorine-containing alicyclic structure may be substituted with chlorine atoms for the purpose of increase in refractive index.

[0028] [in the above formulae (I) to (IV), 1 is from 0 to 5, m is from 0 to 4, n is from 0 to 1, 1+m+n is from 1 to 6, each of o, p and q is from 0 to 5, o+p+q is from 1 to 6, R is F or CF₃, R₁ is F or CF₃, R₂ is F or CF₃, X₁ is F or Cl, and X₂ is F or Cl].

[0029] As the polymer having a fluorine-containing alicyclic structure, polymers having a cyclic structure on their main chains are preferred. Those containing a polymeric unit having a cyclic structure in an amount of at least 20 mol %, preferably at least 40 mol % are preferred in view of transparency and mechanical properties.

[0030] The material (b) is at least one material which differs from the fluoropolymer (a) in refractive index by at least 0.001. It may have a higher refractive index or a lower refractive index than the fluoropolymer (a). In optical fibers, it is usual to use a material having a higher refractive index than the fluoropolymer (a).

[0031] As the material (b), low-molecular weight compounds, oligomers and polymers containing an aromatic ring such as a benzene ring, a halogen atom such as chlorine, bromine or iodine, or a bonding group such as an ether bond are preferred. Further, the material (b) is a material which substantially has no C—H bond for the same reason as the fluoropolymer (a). The difference in refractive index between the fluoropolymer (a) and the material (b) is preferably at least 0.005.

[0032] The oligomeric or polymeric material (b) may be an oligomer or a polymer of such a monomer constituting the fluoropolymer (a) as described above, which differs from the fluoropolymer (a) in refractive index by at least 0.001. Such a monomer is selected from those which form a polymer which differs from the fluoropolymer (a) in refractive index by at least 0.001. For example, it is possible to use two kinds of fluoropolymers (a) having different refractive indices and distribute one polymer (a) in the other polymer (a), as the material (b).

[0033] The material (b) preferably has a solubility parameter within 7(cal/cm³)^(½)of that of the matrix. A solubility parameter is a property value which is a measure of the miscibility between materials. The solubility parameter δ is represented by the formula δ=(E/V)^(½), wherein E is the cohesive energy of a molecule of material, and V is the molar volume.

[0034] As a low-molecular weight compound, halogenated aromatic hydrocarbons having no hydrogen atom bonded to a carbon atom may be mentioned. Halogenated aromatic hydrocarbons containing only fluorine atoms as halogen atoms, and halogenated aromatic hydrocarbons containing a fluorine atom and another halogen atom are preferred in view of the miscibility with the fluoropolymer (a). Among such halogenated aromatic hydrocarbons, those having no functional group such as a carbonyl group or a cyano group are more preferred.

[0035] As such a halogenated aromatic hydrocarbon, a compound represented by the formula Φ_(r)-Z_(b) [wherein Φ_(r) is a b valent fluorinated aromatic ring residue having fluorine atoms substituted for all the hydrogen atoms, and Z is a halogen atom other than fluorine, —Rf, —CO—Rf, —O—Rf or —CN, wherein Rf is a perfluoroalkyl group, a polyfluoroperhaloalkyl group or a monovalent Φ_(r), and b is 0 or an integer of at least l] may, for example, be mentioned. As the aromatic ring, a benzene ring or a naphthalene ring may be mentioned. The carbon number of a perfluoroalkyl group or a polyfluoroperhaloalkyl group as Rf is preferably at most 5. As a halogen atom other than fluorine, a chlorine atom and a bromine atom are preferred.

[0036] As specific compounds, for example, 1,3-dibromotetrafluorobenzene, 1,4-dibromotetrafluorobenzene, 2-bromotetrafluorobenzotrifluoride, chloropentafluorobenzene, bromopentafluorobenzene, iodopentafluorobenzene, decafluorobenzophenone, perfluoroacetophenone, perfluorobiphenyl, chloroheptafluoronaphthalene and bromoheptafluoronaphthalene may be mentioned.

[0037] As the polymeric or oligomeric material (b) among those having the above-mentioned repeating units (I) to (IV), fluoropolymers having a different refractive index from the fluoropolymer (a) to be used in combination (for example, a combination of a fluoropolymer containing fluorine atoms only as halogen atoms with a fluoropolymer containing fluorine atoms and chlorine atoms, and a combination of two kinds of fluoropolymers obtained by polymerizing at least two monomers of different kinds in different proportions) are preferred.

[0038] Further, in addition to the above-mentioned fluoropolymers having a cyclic structure on their main chains, oligomers of monomers containing no hydrogen atom such as tetrafluoroethylene, chlorotrifluoroethylene, dichlorodifluoroethylene, hexafluoropropylene and perfluoroalkyl vinyl ether, and co-oligomers of at least two of these monomers may be used as the material (b). Further, perfluoropolyethers having a structural unit —CF₂CF(CF₃)O— or —(CF₂)_(n)O— (wherein n is an integer of from 1 to 3) may be used. The molecular weights of the oligomers are selected within such a range of molecular weight that they are amorphous, and are preferably from 300 to 10,000 in terms of number-average molecular weight. In view of easy diffusion, it is more preferred that the number-average molecular weights are from 300 to 5000.

[0039] The particularly preferable material (b) is a chlorotrifluoroethylene oligomer since it is excellently compatible with the fluoropolymer (a), particularly with a fluoropolymer having a cyclic structure on its main chain. By virtue of its good compatibility, it is possible to easily mix the fluoropolymer (a), particularly the fluoropolymer having a cyclic structure on its main chain with a chlorotrifluoroethylene oligomer by hot-melting them at 200 to 300° C. It is also possible to homogeneously mix them by dissolving them in a fluorine-containing solvent and then removing the solvent. The preferred molecular weight of a chlorotrifluoroethylene oligomer is from 500 to 1500 in terms of the number-average molecular weight.

[0040] The optical plastic material of the present invention is most preferably a graded-refractive-index optical fiber. In the optical fiber, the material (b) is so distributed in the fluoropolymer (a) as to have a concentration gradient in the direction of from the center to the periphery. Preferably, it is an optical fiber wherein the material (b) is a material having a higher refractive index than the fluoropolymer (a), and the material (b) is so distributed as to have such a concentration gradient that the concentration of the material (b) decreases in the direction of from the center of the optical fiber to the periphery. In some cases, an optical fiber wherein the material (b) is a material having a lower refractive index than the fluoropolymer (a), and the material (b) is so distributed as to have a concentration gradient that the concentration of the material (b) decreases in the direction of from the periphery of the optical fibers the center, is also useful. A light transmission medium such as the former optical fiber is usually produced by arranging the material (b) at the center and diffusing the material (b) toward the periphery. A light transmission medium such as the latter optical fiber is produced by diffusing the material (b) from the periphery toward the center.

[0041] A light transmission medium which is the optical plastic material of the present invention has a transmission loss per 100 m of at most 1000 dB at wavelengths of from 700 to 1,600 nm. Particularly, when a fluoropolymer having an alicyclic structure on its main chain is used, it has a transmission loss per 100 m of at most 50 dB. It is quite advantageous that the transmission loss is at such a low level at relatively long wavelengths of from 700 to 1,600 nm. Namely, it has advantages that since it is available to the same wavelength as vitreous silica optical fiber, it can be connected to a vitreous silica optical fiber without any difficulties, and that a cheaper light source can be used as compared with the case of conventional plastic optical fibers which are available only to light at wavelengths shorter than from 700 to 1,600 nm.

[0042] In production of the optical plastic material of the present invention, the molding of the resins and the formation of the graded refractive index may be carried out simultaneously or separately. For example, the optical plastic material of the present invention may be so produced by spinning or extrusion molding that a graded refractive index is formed at the same time as formation of a graded refractive index. It is also possible to form a graded refractive index after molding the resins by spinning or extrusion molding. Further, it is possible to produce a preform (body material) having a graded refractive index and then form (for example spin) the preform into an optical plastic material such as an optical fiber. As described above, the optical plastic material of the present invention also means such a preform having a graded refractive index.

[0043] As a method of producing the optical plastic material of the present invention, for example, the following methods (1) to (7) may be mentioned. However, the present invention is not limited to these methods. The method (1) is particularly preferred.

[0044] (1) A method which comprises melting the fluoropolymer (a), injecting the material (b) or a fluoropolymer (a) containing the material (b) at the center of the melt of the fluoropolymer (a), and then molding the melt while or after diffusing the material (b).

[0045] In this case, the material (b) may be injected at the center not only so as to form only one layer but also so as to form multiple layers. The molding is carried out by melt-extrusion, which is suitable for forming a rod-like body material such as a preform of an optical fiber, or by melt-spinnig, which is suitable for forming an optical fiber.

[0046] (2) A method which comprises dip-coating the material (b) or the fluoropolymer (a) containing the material (b) on a core formed from the fluoropolymer (a) by melt spinning or drawing.

[0047] (3) A method which comprises forming a hollow tube of the fluoropolymer (a) by using a rotating glass tube or the like, filling in the polymer tube with a monomer phase which gives the material (b) or the fluoropolymer (a) which contains the material (b), and then polymerizing the monomer phase while rotating the polymer tube at a low speed.

[0048] In the case of interfacial gel polymerization, at the polymerization step, the tube of the fluoropolymer (a) swells up in the monomer phase and forms a gel phase, and the monomer molecules are polymerized while preferentially diffusing in the gel phase.

[0049] (4) A method wherein two kinds of monomers with different reactivities, one of which is a monomer which forms the fluoropolymer (a), and the other is a monomer which forms the material (b), are used, and the polymerization reaction is carried out so that the compositional proportion of the resulting fluoropolymer (a) to the resulting material (b) varies continuously in the direction from the periphery to the center.

[0050] (5) A method which comprises hot-drawing or melt-extruding a mixture of the fluoropolymer (a) and the material (b) obtained by homogeneously mixing them or by homogeneously mixing them in a solvent and then removing the solvent upon evaporation, into fibers, and then (or immediately after the formation of the fibers) bringing the fibers into contact with an inert gas under heating to evaporate the material (b) from the surface and thereby forming a graded refractive index. Or, a method wherein after the formation of the fibers, the fibers are immersed in a solvent which does not dissolve the fluoropolymer (a) but dissolves the material (b) so as to dissolve out the material (b) from the surface of the fibers so that a graded refractive index is formed.

[0051] (6) A method which comprises coating a rod or a fiber of the fluoropolymer (a) with only the material (b) which has a smaller refractive index than the fluoropolymer (a) or with a mixture of the fluoropolymer (a) and the material (b), and then diffusing the material (b) by heating to form a graded refractive index.

[0052] (7) A method which comprises mixing a high-refractive-index polymer and a low-refractive-index polymer by hot-melting or in a state of a solution containing a solvent, and diffusing them in each other while (or after) multilayer-excluding in a state that each has a different mixing ratio, to eventually obtain a fiber having a graded refractive index. In this case, the high-refractive-index polymer may be the fluoropolymer (a), and the low-refractive-index polymer may be the material (b). The high-refractive-index polymer is the material (b), and the low-refractive-index polymer is the material (b).

[0053] In the present invention, by virtue of the application of an amorphous fluoro resin to various plastic light transmission medium such as a graded-refractive-index optical fiber, a graded-refractive-index optical waveguide and a graded-refractive-index rod lens, it is possible to transmit light ranging from ultraviolet light to near infrared light with a quite low loss.

[0054] A graded-refractive-index optical fiber is particularly suitable for optical communication over short distances in spite of its large diameter since it is flexible and it is easy to form branches and junctions or it. However, no practical optical fiber with a low loss has been proposed so far. The present invention provides a practical optical fiber with a low loss for optical communication over short distances.

[0055] The light transmission medium of the present invention provides a plastic light transmission medium having heat resistance, chemical resistance, humidity resistance and nonflammability enough to withstand severe use conditions in an engine room of an automobile, an OA equipment, a plant and an electrical appliance. The graded-refractive-index optical plastic material of the present invention can be used not only as an optical fiber but also as a flat or rod lens. In such a case, by increasing or decreasing the refractive index from the center to the periphery, it can function as a convex lens or a concave lens.

BRIEF DESCRIPTION OF DRAWING

[0056]FIG. 1 is the transmittance of polymer A.

BEST MODE FOR CARRYING OUT THE INVENTION

[0057] The present invention will be described in detail with reference to Examples. However, it should be understood that the present invention is by no means restricted to such specific Examples.

SYNTHESIS EXAMPLE 1

[0058] 35 g of perfluoro(butenyl vinyl ether) [PBVE], 5 g of 1,1,2-trichlorotrifluoroethane (R113), 150 g of deionized water and 90 mg of ((CH₃)₂CHOCOO)₂ as a polymerization initiator, were put in a pressure glass autoclave of an internal volume of 200 ml. The atmosphere in the autoclave was replaced by nitrogen three times, and suspension polymerization was conducted at 40° C. for 22 hours, yielding 28 g of a polymer having a number-average molecular weight of about 1.5×10⁵ (hereinafter referred to as polymer A).

[0059] The intrinsic viscosity [η] of polymer A, measured in perfluoro(2-butyltetrahydrofuran) [PBTHF] at 30° C., was 0.50. The glass transition point of polymer A was 108° C., and it was a tough, transparent and glassy polymer. The 10% thermal decomposition temperature was 465° C., the solubility parameter was 5.3 cal/cm³, and the refractive index was 1.34. FIG. 1 illustrates the transmittance of polymer A.

SYNTHESIS EXAMPLE 2

[0060] Perfluoro(2,2-dimethyl-1,3-dioxole) [PDD] and tetrafluoroethylene in the weight ratio of 80:20 were radical polymerized, and thereby a polymer having a glass transition point of 160° C. and a number-average molecular weight of about 5×10⁵ (hereinafter referred to as polymer B) was obtained. Polymer B was colorless and transparent, and had a refractive index of 1.3 and a high transmittance.

[0061] PDD and chlorotrifluoroethylene (CTFE)in the weight ratio of 75:25 were radical polymerized, and thereby a polymer having a glass transition point of 150° C. and a number-average molecular weight of about 3×10⁵ (hereinafter referred to as polymer C) was obtained. Polymer C was colorless and transparent, and had a refractive index of 1.4 and a high transmittance.

SYNTHESIS EXAMPLE 3

[0062] 8 g of PBVE, 2 g of PDD, 10 g of PBTHF and 20 mg of ((CH₃)₂CHOCOO)₂ as a polymerization initiator were put in a pressure glass ampoule of an internal volume of 50 ml. The atmosphere in the ampoule was replaced by nitrogen three times, and polymerization was conducted at 40° C. for 20 hours, yielding 6.7 g of a transparent polymer having a number-average molecular weight of about 2×10⁵ (hereinafter referred to as polymer D).

[0063] The glass transition point of polymer D was 157° C., the refractive index was 1.32, and the content of PDD polymeric unit determined by measuring the absorbance at 1930 cm⁻¹ on the IR spectrum, was 12 wt %.

[0064] 2 g of PBVE, 8 g of PDD, 10 g PBTHF and 20 mg of ((CH₃)₂CHOCOO)₂ as a polymerization initiator, were put in a pressure glass ampoule of an internal volume of 50 ml. The ampoule was freeze-degassed three times, and then polymerization was conducted at 30° C. for 20 hours. Thereby, 7 g of a transparent polymer having a number-average molecular weight of about 3×10⁵ (hereinafter referred to as polymer E) was obtained.

[0065] The glass transition point of polymer E was 210° C., the refractive index was 1.29, the content of PDD polymeric unit determined by measuring the absorbance at 1930 cm⁻¹ on the IR spectrum, was 82 wt %.

EXAMPLE 1

[0066] Polymer A obtained by the above Synthesis was dissolved in PBTEF solvent, and then 12 wt % of 1,3-dibromotetrafluorobenzene (DBTFB), which had a refractive index of 1.52 and was different from polymer A in solubility parameter by 3.2 cal/cm³, was added to obtain a mixed solution. From the solution, the solvent was removed, to obtain a transparent mixed polymer (hereinafter referred to as polymer F).

[0067] Polymer A was melted, and melt-spinning was conducted at 300° C. while the melt of polymer F was injected at the center of the melt of polymer A, thereby an optical fiber having a refractive index gradually decreasing in the direction of from the center to the periphery, was obtained.

[0068] The light transmission property of the optical fiber thus obtained was 300 dB/km at 780 nm, and 130 dB/km at 1550 nm. The optical fiber was confirmed to be capable of transmit light ranging from visible light to near infrared light satisfactorily.

EXAMPLE 2

[0069] 40 g of PBVE and 500 ml of ((CH₃)₂CHOCOO)₂ as a polymerization initiator were introduced into glass tube. After the tube was freeze-degassed, polymerization was conducted while the glass tube was rotated at a high speed. The hollow tube thus synthesized was removed from the glass tube to obtain a tube of a polymer having a number-average molecular weight of about 1×10⁵. The tube was charged with 20 g of PBVE, 2 g of DBTFB as a high-refractive-index material and 200 ml of ((CH₃)₂CHOCOO)₂ as a polymerization initiator, and then sealed, and polymerization was conducted while the tube was rotated at a low speed.

[0070] At the polymerization step, the polymer of the tube swells up in the monomer phase, forming a gel phase. The polymerization in the gel phase is promoted by the gel effect, and the polymer phase is formed from the periphery. At this time, the monomer molecules preferentially diffuse in the gel phase since monomer molecules are smaller in size than molecules of the high-refractive-index material, and the polymerization proceeds with the high refractive index material concentrated at the center, to form such a graded refractive index that the refractive index gradually decreases from the center to the periphery. The preform thus obtained as hot-drawn to obtain a graded-refractive-index optical fiber.

[0071] The light transmission property of the optical fiber thus obtained was 500 dB/km at 650 nm, and 150 dB/km at 1550 nm. The optical fiber was confirmed to be capable of transmitting light ranging from visible light to near infrared light satisfactorily.

EXAMPLE 3

[0072] A core of 30μ was prepared from polymer D obtained in the above Synthesis. On the other hand, a solution containing polymer D at a concentration of 1 wt % in PBTHF solvent (hereinafter referred to as solution D) was prepared. Similarly, a solution containing 1 wt % of polymer E in PBTHF solvent (hereinafter referred to as solution E) was prepared. Solution D was dip-coated on the core of polymer D at a drawing rate of 6 cm, and then dried at 180° C. It was found that the diameter of the core of polymer D increased by 100 nm.

[0073] Dip coating and drying were repeated 500 times while adding a 1/250 portion by weight of solution E to the solution D each time. Finally, solution E at a concentration of 10 wt % was dip-coated and dried repeatedly five times, and it was dried at 180° C. for 2 hours. Thereby, an optical fiber having a diameter of about 600μ wherein the refractive index gradually decreased in a direction of from the core to the periphery, was obtained.

[0074] The light transmission property of the optical fiber thus obtained was 1050 dB/km at 650 nm, 460 dB/km at 950 nm, and 130 dB/km at 1300 nm. The optical fiber was confirmed to be capable of transmitting light ranging from visible light to near infrared light satisfactorily.

EXAMPLE 4

[0075] Equal weights of polymer B and polymer C synthesized above were dissolved in PBTHF solvent and mixed. From the resulting solution, the solvent was removed to obtain a transparent polymer mixture (B+C). A melt of polymer mixture (B+C) was poured inside a melt of polymer B, and further a melt of polymer C was injected at the center, while being melt spun, to obtain an optical fiber which has a refractive index gradually decreasing from the center to the periphery.

[0076] The light transmission property of the optical fiber thus obtained was 550 dB/km at 650 nm, and 130 dB/km at 1550 nm. The optical fiber was confirmed to be capable of transmitting light ranging from visible light to near infrared light satisfactorily.

EXAMPLE 5

[0077] An optical fiber was obtained in the same manner as in Example 1 except that 30 wt % of a CTFE oligomer having a number-average molecular weight of 800 was used instead of 12 wt % of DBTFB. The refractive index of the oligomer was 1.41, and the difference in solubility parameter between the oligomer and the polymer A was 1.4 cal/cm³. The optical fiber thus obtained had such a refractive index as decreased gradually from the center to the periphery.

[0078] The light transmission property of the optical fiber was 280 dB/km at 780 nm, and 120 dB/km at 1550 nm. The optical fiber was confirmed to be capable of transmitting light ranging from visible light to near infrared light satisfactorily.

EXAMPLE 6

[0079] 50 Parts of PDD having a reactivity ratio r¹ (the ratio of the rate of production of PDD homopolymer to the ratio of production of PDD/PBVE copolymer) of 1.9, 50 parts of PBVE having a reactivity ratio r² (the ratio of the rate of production of PBVE homopolymer to the ratio of production of PDD/PBVE copolymer) of 0.19, and 1 part of dialkoxyacetophenone as a photoinitiator, dissolved in 5 parts of HCFC25, were introduced in a glass ampoule. After the ampoule was freeze-degassed three times, photopolymerization was conducted by using a low-pressure mercury lamp. Thereby, a preform having a graded refractive index with a refractive index at the periphery of 1.31 and a refractive index at the center of 1.33, was obtained. It was hot-drawn to obtain a graded-refractive-index optical fiber.

[0080] The light transmission property of the optical fiber was 320 dB/Km at 650 nm, and 250 dB/Km at 1550 nm. The optical fiber was confirmed to be capable of transmitting light ranging from visible light to ultraviolet light satisfactorily.

EXAMPLE 7

[0081] 85 Parts of polymer A and 15 parts of DBTFB were melt-mixed and formed into a rod. The rod was hot-drawn at 200° C. to obtain a fiber. At this time, after emerged from the hot drawing region, the fiber was passed through an electric oven of 1 m long at 120° C. In the electric oven, a flow of dry air preliminary heated to 120° C. was made to evaporate DBTFB from the surface of the fiber and thereby obtain an optical fiber having a graded refractive index.

[0082] The light transmission property of the optical fiber was 420 dB/km at 650 nm, 250 dB/km at 780 nm, and 110 dB/km at 1300 nm. The optical fiber was confirmed to be capable of transmitting light ranging from visible light to near infrared light satisfactorily.

EXAMPLE 8

[0083] 90 Parts of PBVE and 10 parts of CTFE were polymerized to obtain a polymer having a number-average molecular weight of about 2×10⁵ (hereinafter referred to as polymer F). Polymer F was melt-mixed with a CTFE oligomer having a number-average molecular weight of 800 homogeneously, and the melt mixture was formed into a rod containing the oligomer in an amount of 20 wt %.

[0084] The rod was hot-drawn into a fiber of 500μ diameter. The fiber was passed through ethanol to dissolve out the CTFE oligomer, and then passed through a cylindrical hot oven at 20° C. with a residence time of about 10 seconds for drying. Thereby, a graded-refractive-index optical fiber with a refractive index at the periphery of 1.36 and a refractive index at the center of 1.38, was obtained.

[0085] The light transmission property of the optical fiber thus obtained was 250 dB/Km at 650 nm, and 150 dB/Km at 1550 nm. The optical fiber was confirmed to be capable of transmitting light ranging from visible light to ultraviolet light satisfactorily.

EXAMPLE 9

[0086] Polymer C was spun at 270° C. by a extrusion method, and the resulting fiber was immediately passed through a hexafluoropropylene oxide (HFPO) oligomer (number-average molecular weight 2100) heated to 220° C. so that the residence time would be 3 minutes. As a result, the HFPO oligomer diffused and penetrated into the fiber, thereby, an optical fiber having an outer diameter of 600μ and a refractive index continuously varying in the direction of from the periphery to the center, was obtained. The refractive index at the periphery was 1.34, and the refractive index at the center was 1.35.

[0087] The light transmission property of the optical fiber thus obtained was 300 dB/Km at 650 nm, and 130 dB/Km at 1550 nm. The optical fiber was confirmed to be capable of transmitting light ranging from visible light to ultraviolet light satisfactorily.

EXAMPLE 10

[0088] A polymer having a PDD content of 20 wt % and a number-average molecular weight of about 1×10⁵ (hereinafter referred to as polymer G) and a polymer having a PDD content of 60 wt % and a number-average molecular weight of about 5×10⁵ (hereinafter referred to as polymer H) were synthesized by polymerization of PDD and PBVE. The refractive indices were 1.33 for polymer G and 1.31 for polymer H, respectively.

[0089] Polymers G and H were dissolved in perfluorotributylamine/perfluorooctane=20/80 (weight ratio), respectively, so that the polymer concentrations would be 20 wt %. Then, the resulting solutions were mixed in the ratios shown in Table 1 to prepare 11 kinds of solutions, and they were heated to evaporate part of the solvent. Thereby gel solutions of about 3000 cP were obtained. The 11 kinds of gels having different mixing ratios were extruded concentrically by using a multilayer nozzle into a multilayer fiber, while being heated at 80° C. The fiber was passed through a hot oven (about 150 to 200° C.) in which air flow was made to remove the residual solvent. Thereby, a graded-refractive-index fiber was obtained.

[0090] The light transmission property of the optical fiber thus obtained was 350 dB/km at 650 nm, 150 dB/km at 950 nm, and 120 dB/km at 1300 nm. The optical fiber was confirmed to be capable of transmitting light ranging from visible light to near infrared light satisfactorily.

COMPARATIVE EXAMPLE

[0091] As for a graded-refractive-index plastic optical fiber, the light transmission loss of PMMA was about 400 dB/km at 650nm, and the transmission losses at wavelengths of 780 nm, 1300 nm and 1550 nm were so large that it was impractical as a light transmission medium.

[0092] A stepped-refractive-index plastic optical fiber having a core and a cladding made of fluoro resins is reported to be capable of transmitting light ranging from visible light to near infrared light but has a light transmission loss of about 300 dB/km.

[0093] By contrast, the graded-refractive-index transparent fluoro resin optical fiber of the present invention is capable of transmit light ranging from visible light to near infrared light with extremely low losses. TABLE 1 Polymer G Polymer H 100 parts 0 part 81 19 64 36 49 51 36 64 25 75 16 84 9 91 4 96 1 99 0 100 

1. A graded-refractive-index optical plastic material composed of an amorphous fluoropolymer (a) which substantially has no C—H bond and at least one material (b) which differs from the fluoropolymer (a) in refractive index by at least 0.001, wherein the material (b) is distributed in the fluoropolymer (a) so as to have a concentration gradient in a specific direction.
 2. The optical plastic material according to claim 1 , wherein the fluoropolymer (a) is a fluoropolymer having a cyclic structure on its main chain.
 3. The optical plastic material according to claim 2 , wherein the fluoropolymer having a cyclic structure on its main chain is a fluoropolymer having a fluorine-containing alicyclic structure on its main chain.
 4. The optical plastic material according to claim 3 , wherein the fluoropolymer having a fluorine-containing alicyclic structure on its main chain has a repeating unit selected from the following formulae (I) to (IV):

[in the above formulae (I) to (IV), 1 is from 0 to 5, m is from 0 to 4, n is from 0 to 1, 1+m+n is from 1 to 6, each of o, p and q is from 0 to 5, o+p+q is from 1 to 6, R is F or CF₃, R₁ is F or CF₃, R₂ is F or CF₃, X₁ is F or Cl, and X₂ is F or Cl].
 5. The optical plastic material according to claim 1 , wherein the fluoropolymer (a) has a melt viscosity of from 10³ to 10⁵ poise at a melt temperature of from 200 to 300° C.
 6. The optical plastic material according to claim 1 , wherein the material (b) substantially has no C—H bond.
 7. The optical plastic material according to claim 1 , wherein the material (b) differs from the fluoropolymer (a) in solubility parameter by at most 7(cal/cm³)^(½).
 8. The optical plastic material according to claim 1 , wherein the material (b) has a molecular weight of from 300 to
 5000. 9. The optical plastic material according to claim 1 , wherein the material (b) is a chlorotrifluoroethylene oligomer.
 10. The optical plastic material according to claim 1 , which is a graded-refractive-index optical fiber.
 11. A graded-refractive-index optical fiber which is the optical plastic material according to claim 1 , wherein the material (b) is so distributed as to have a concentration gradient in a direction of from the center to the periphery.
 12. The graded-refractive-index optical fiber according to claim 11 , wherein the material (b) has a higher refractive index than the fluoropolymer (a), and is so distributed as to have such a concentration gradient that the concentration decreases in a direction of from the center to the periphery.
 13. A method for producing a graded-refractive-index optical plastic material, which comprises melting an amorphous fluoropolymer (a) which substantially has no C—H bond, injecting at least one material (b) which differs in refractive index by at least 0.001 from the fluoropolymer (a) or the fluoropolymer (a) containing the material (b) at the center of the melt of the fluoropolymer (a), and molding the melt while or after diffusing the material (b) to form a region wherein the refractive index varies continuously.
 14. The method according to claim 13 , wherein the fluoropolymer (a) is a fluoropolymer having a cyclic structure on its main chain.
 15. The method according to claim 13 , wherein the material (b) substantially has no C—H bond.
 16. The method according to claim 13 , wherein the molding is conducted by melt extrusion molding or by melt spinning molding. 